The proposed research is an extension of our work on the factors which control transepithelial calcium and phosphate movement. In addition to vitamin D and parathyroid hormone, intestinal transport of calcium is modified by the concentration of sodium and other monovalent cations in the fluid bathing the mucosa. We have developed an in vitro system in which uptake of calcium by intestinal mucosa can be measured in the absence of net transport of calcium into the serosal compartment. The influence of the concentration of sodium and other monovalent cations and of inhibitors of sodium transport on this mocosal uptake and on the efflux of calcium from tissue into mucosal medium will be determined. Active transport of phosphate in vitro by intestine of rats made hypophosphatemic by low phosphate diet is greater than that of intestine from normophosphatemic rats. The mechanism of this adaptation of the phosphate transport system in relation to vitamin D action will be examined. The phenomenon of increased vitamin D requirement has been observed in patients receiving anticonvulsants. We wish to measure the effects of phenobarbital, dilantin and primidone on the rates of disappearance of the physiologic activity of vitamin D and vitamin D metabolites in rats and their action on the calcium and phosphate tranport systems of the intestine to determine the mechanism of the anti-vitamin D activity of these drugs.